The present invention generally relates to a laser processing apparatus for processing articles using a laser beam, and more particularly, to a laser processing technique applied to fabrication of high-precision components requiring a resolution of several microns to several hundred nanometers, as well as two-dimensional or three-dimensional shape-processed devices (such as micro-electric-mechanical systems (MEMS) devices or diffractive optics) and components with a number of fine holes, such as photonic crystals, printed boards, or inkjet heads.
Because a laser beam can converge accurately on a fine area in short time, it is used in various types of processing, including direct material processing and indirect processing making use of chemical change, reformation or alteration of the material. Direct processing includes surface patterning, drilling, ablation, and cutting of metals or synthetic resins. Examples of indirect processing include lithography and optical molding. Especially, in recent years and continuing, laser processing is often employed in superfine two-dimensional or three-dimensional shape processing to fabricate refractive and diffractive optical devices, master disks of optical disks, electric circuits, MEMS devices, etc.
Conventionally, there are several methods for performing shape processing using a laser beam, including:    (1) focusing the beam directly onto the object to be processed, while moving the focal point or the object (relative to each other); and    (2) guiding the beam through a mask to project the mask pattern onto the surface to be processed.
The first method (1) is grouped into two categories depending on which one of the object and the beam is moved, that is, moving the object to be processed in a direction perpendicular to the optical axis of the laser beams, or scanning the focal point using a galvano scanner. The method using the galvano scanner generally achieves faster and more precise processing; however, the throughput falls when surface processing or pattern processing over a wide area is performed.
The second method (2) allows collective processing of highly precise patterns by scaling down the mask pattern and projecting the downscaled pattern onto the surface to be processed. In addition, three-dimensional processing is possible using a gray scale mask. However, this method has the following problems:    (a) Since the light beam is blocked by a mask, light use efficiency becomes very low depending on the pattern to be projected; and    (b) Variation in light intensity guided to the mask leads to unevenness of the resultant shape of the processed products.
To solve these technical problems, it is proposed to use a diffraction optical device or a hologram to perform collective shape processing at high light use efficiency. See, for example, JP 2002-66769A and JP 2001-272635A.
With this method, the light beam emitted from a coherent source is shaped using a device for modulating the phase and/or the amplitude of the light beam in order to acquire a desired beam shape on the processing surface.
By the way, it is required for a laser processing apparatus to perform positioning of the beam onto a processed site. Especially when performing high-resolution shape processing, positioning accuracy at or below one tenth of the resolution is required.
Conventionally, various methods for positioning the focusing point of the laser beam onto a desired position have been proposed, for example, by using a closed-loop type stage with position detecting equipment or a piezo element, or using a high angular-resolution galvano scanner. The above-described JP 2002-66769 discloses a combination of the hologram device and a galvano scanner to achieve precise shape processing over a wide area.
However, such a laser processing apparatuses is expensive, and in addition, it causes vibration due to mechanical movement of the movable part or element.
Another problem is that when the processing position moves in the depth direction to process a three-dimensional shape or to form a hole, the focal point has to be controlled along the optical axis of the laser beam during the processing.
A conventional focal point control method employs an actuator using a piezo element as the driving source in the focusing optical system, or the stage on which a sample is fixed is driven.
With either method the apparatus become expensive because a highly precise driving mechanism is required, and in spite of the high-cost apparatus, the positioning accuracy is degraded due to vibration due to acceleration or slowdown.
Other prior art publications in the laser processing field are JP 2000-223766A and JP 2001-209003A. The former publication is directed to a laser processing apparatus in which the phase of the laser beam is modulated by modulation means structured by a liquid crystal panel so as to change the focal position or the beam shape. The latter publication is directed to a laser processing apparatus having a laser source, an optical system for collimating the laser beam emitted from the laser source into a parallel beam, and a spot shape transforming device for guiding the parallel beam onto the surface to be processed with a desired intensity distribution profile. The spot shape transforming device has functions of a lens and a spot shaper, and it controls the direction of the beam and redistributes the beam intensity so as to achieve a prescribed intensity distribution on the target surface. These two publications simply disclose known laser beam control techniques in the laser processing field, and further explanation is withheld.